This document discusses network configuration and diagnostic tools in Linux. It describes how to view interface configuration and address information using ifconfig and ip addr show. It also covers changing IP and route settings persistently in configuration files and using ifup/ifdown. Tools like ping and traceroute are presented for network diagnostics.

1. Chapter 18Chapter 18
Network ConfigurationNetwork Configuration

2. Network Devices in LinuxNetwork Devices in Linux
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lolo
– Loop back deviceLoop back device
– IP address is 127.0.0.1 permanentlyIP address is 127.0.0.1 permanently
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ethethNN
– Ethernet interfaceEthernet interface
– Device number starting from 0Device number starting from 0
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wlanwlanNN
– Wireless interfaceWireless interface
– Device number starting from 0Device number starting from 0

3. Display Interface ConfigurationDisplay Interface Configuration
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ifconfigifconfig
eth0 Link encap:Ethernet HWaddr 00:16:d3:32:ea:dceth0 Link encap:Ethernet HWaddr 00:16:d3:32:ea:dc
inet addr:192.168.100.57 Bcast:192.168.100.255 Mask:255.255.255.0inet addr:192.168.100.57 Bcast:192.168.100.255 Mask:255.255.255.0
inet6 addr: fe80::216:d3ff:fe32:eadc/64 Scope:Linkinet6 addr: fe80::216:d3ff:fe32:eadc/64 Scope:Link
UP BROADCAST RUNNING MULTICAST MTU:1500 Metric:1UP BROADCAST RUNNING MULTICAST MTU:1500 Metric:1
RX packets:18973 errors:0 dropped:0 overruns:0 frame:0RX packets:18973 errors:0 dropped:0 overruns:0 frame:0
TX packets:16624 errors:0 dropped:0 overruns:0 carrier:0TX packets:16624 errors:0 dropped:0 overruns:0 carrier:0
collisions:0 txqueuelen:1000collisions:0 txqueuelen:1000
RX bytes:12613255 (12.6 MB) TX bytes:2686873 (2.6 MB)RX bytes:12613255 (12.6 MB) TX bytes:2686873 (2.6 MB)
Interrupt:16 Memory:ee000000-ee020000Interrupt:16 Memory:ee000000-ee020000
lo Link encap:Local Loopbacklo Link encap:Local Loopback
inet addr:127.0.0.1 Mask:255.0.0.0inet addr:127.0.0.1 Mask:255.0.0.0
inet6 addr: ::1/128 Scope:Hostinet6 addr: ::1/128 Scope:Host
UP LOOPBACK RUNNING MTU:65536 Metric:1UP LOOPBACK RUNNING MTU:65536 Metric:1
RX packets:523 errors:0 dropped:0 overruns:0 frame:0RX packets:523 errors:0 dropped:0 overruns:0 frame:0
TX packets:523 errors:0 dropped:0 overruns:0 carrier:0TX packets:523 errors:0 dropped:0 overruns:0 carrier:0
collisions:0 txqueuelen:0collisions:0 txqueuelen:0
RX bytes:38418 (38.4 KB) TX bytes:38418 (38.4 KB)RX bytes:38418 (38.4 KB) TX bytes:38418 (38.4 KB)

4. Display Address InformationDisplay Address Information
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ip addr showip addr show
1: lo: <LOOPBACK,UP,LOWER_UP> mtu 65536 qdisc noqueue state1: lo: <LOOPBACK,UP,LOWER_UP> mtu 65536 qdisc noqueue state
UNKNOWNUNKNOWN
link/loopback 00:00:00:00:00:00 brd 00:00:00:00:00:00link/loopback 00:00:00:00:00:00 brd 00:00:00:00:00:00
inet 127.0.0.1/8 scope host loinet 127.0.0.1/8 scope host lo
valid_lft forever preferred_lft forevervalid_lft forever preferred_lft forever
inet6 ::1/128 scope hostinet6 ::1/128 scope host
valid_lft forever preferred_lft forevervalid_lft forever preferred_lft forever
2: eth0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc pfifo_fast2: eth0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc pfifo_fast
state UP qlen 1000state UP qlen 1000
link/ether 00:16:d3:32:ea:dc brd ff:ff:ff:ff:ff:fflink/ether 00:16:d3:32:ea:dc brd ff:ff:ff:ff:ff:ff
inet 192.168.100.57/24 brd 192.168.100.255 scope global eth0inet 192.168.100.57/24 brd 192.168.100.255 scope global eth0
valid_lft forever preferred_lft forevervalid_lft forever preferred_lft forever
inet6 fe80::216:d3ff:fe32:eadc/64 scope linkinet6 fe80::216:d3ff:fe32:eadc/64 scope link
valid_lft forever preferred_lft forevervalid_lft forever preferred_lft forever

5. Display Link InformationDisplay Link Information
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ip link showip link show
1: lo: <LOOPBACK,UP,LOWER_UP> mtu 65536 qdisc noqueue state1: lo: <LOOPBACK,UP,LOWER_UP> mtu 65536 qdisc noqueue state
UNKNOWNUNKNOWN
link/loopback 00:00:00:00:00:00 brd 00:00:00:00:00:00link/loopback 00:00:00:00:00:00 brd 00:00:00:00:00:00
2: eth0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc pfifo_fast2: eth0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc pfifo_fast
state UP qlen 1000state UP qlen 1000
link/ether 00:16:d3:32:ea:dc brd ff:ff:ff:ff:ff:fflink/ether 00:16:d3:32:ea:dc brd ff:ff:ff:ff:ff:ff

6. Display Link CharacteristicsDisplay Link Characteristics
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ip ­s link showip ­s link show
1: lo: <LOOPBACK,UP,LOWER_UP> mtu 65536 qdisc noqueue state1: lo: <LOOPBACK,UP,LOWER_UP> mtu 65536 qdisc noqueue state
UNKNOWNUNKNOWN
link/loopback 00:00:00:00:00:00 brd 00:00:00:00:00:00link/loopback 00:00:00:00:00:00 brd 00:00:00:00:00:00
RX: bytes packets errors dropped overrun mcastRX: bytes packets errors dropped overrun mcast
38418 523 0 0 0 038418 523 0 0 0 0
TX: bytes packets errors dropped carrier collsnsTX: bytes packets errors dropped carrier collsns
38418 523 0 0 0 038418 523 0 0 0 0
2: eth0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc pfifo_fast2: eth0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc pfifo_fast
state UP qlen 1000state UP qlen 1000
link/ether 00:16:d3:32:ea:dc brd ff:ff:ff:ff:ff:fflink/ether 00:16:d3:32:ea:dc brd ff:ff:ff:ff:ff:ff
RX: bytes packets errors dropped overrun mcastRX: bytes packets errors dropped overrun mcast
12629558 19078 0 0 0 24512629558 19078 0 0 0 245
TX: bytes packets errors dropped carrier collsnsTX: bytes packets errors dropped carrier collsns
2711702 16782 0 0 0 02711702 16782 0 0 0 0

7. Display Routing InformationDisplay Routing Information
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route ­nroute ­n
Kernel IP routing tableKernel IP routing table
Destination Gateway Genmask Flags Metric Ref Use IfaceDestination Gateway Genmask Flags Metric Ref Use Iface
0.0.0.0 192.168.100.11 0.0.0.0 UG 0 0 0 eth00.0.0.0 192.168.100.11 0.0.0.0 UG 0 0 0 eth0
169.254.0.0 0.0.0.0 255.255.0.0 U 1000 0 0 eth0169.254.0.0 0.0.0.0 255.255.0.0 U 1000 0 0 eth0
192.168.100.0 0.0.0.0 255.255.255.0 U 1 0 0 eth0192.168.100.0 0.0.0.0 255.255.255.0 U 1 0 0 eth0

8. Display Routing InformationDisplay Routing Information
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ip route showip route show
default via 192.168.100.11 dev eth0 proto staticdefault via 192.168.100.11 dev eth0 proto static
169.254.0.0/16 dev eth0 scope link metric 1000169.254.0.0/16 dev eth0 scope link metric 1000
192.168.100.0/24 dev eth0 proto kernel scope link src 192.168.100.57192.168.100.0/24 dev eth0 proto kernel scope link src 192.168.100.57
metric 1metric 1

9. Display DNS SettingsDisplay DNS Settings
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cat /etc/resolv.confcat /etc/resolv.conf
cat /etc/resolv.confcat /etc/resolv.conf
# Dynamic resolv.conf(5) file for glibc resolver(3) generated by resolvconf(8)# Dynamic resolv.conf(5) file for glibc resolver(3) generated by resolvconf(8)
# DO NOT EDIT THIS FILE BY HAND -- YOUR CHANGES WILL BE# DO NOT EDIT THIS FILE BY HAND -- YOUR CHANGES WILL BE
OVERWRITTENOVERWRITTEN
nameserver 192.168.100.11nameserver 192.168.100.11

10. Change IP SettingsChange IP Settings
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Add IP:Add IP:
– ifconfig eth0:0 IP/MASKifconfig eth0:0 IP/MASK
– ip address add IP/MASK dev eth0ip address add IP/MASK dev eth0
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Delete IP:Delete IP:
– ifconfig eth0:0 downifconfig eth0:0 down
– ip address del IP/MASK dev eth0ip address del IP/MASK dev eth0

11. Change Route SettingsChange Route Settings
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Add route:Add route:
– route add ­net NET/MASK gw GW_IProute add ­net NET/MASK gw GW_IP
– ip route add NET/MASK via GW_IPip route add NET/MASK via GW_IP
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Delete route:Delete route:
– route del ­net NET/MASK gw GW_IProute del ­net NET/MASK gw GW_IP
– ip route del NET/MASK via GW_IPip route del NET/MASK via GW_IP

12. Persistent IP SettingsPersistent IP Settings
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Redhat/CentOS:Redhat/CentOS:
vi /etc/sysconfig/network­scripts/eth0vi /etc/sysconfig/network­scripts/eth0
# Intel Corporation 82545EM Gigabit Ethernet # Intel Corporation 82545EM Gigabit Ethernet
Controller (Copper)Controller (Copper)
DEVICE=eth0DEVICE=eth0
BOOTPROTO=noneBOOTPROTO=none
HWADDR=00:0c:29:05:e1:d2HWADDR=00:0c:29:05:e1:d2
IPADDR=163.16.5.180IPADDR=163.16.5.180
NETMASK=255.255.255.0NETMASK=255.255.255.0
ONBOOT=yesONBOOT=yes
GATEWAY=163.16.5.254GATEWAY=163.16.5.254
TYPE=EthernetTYPE=Ethernet
USERCTL=noUSERCTL=no
IPV6INIT=noIPV6INIT=no
PEERDNS=noPEERDNS=no

13. Persistent IP SettingsPersistent IP Settings
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Ubuntu:Ubuntu:
vi /etc/network/interfacesvi /etc/network/interfaces
auto loauto lo
iface lo inet loopbackiface lo inet loopback
iface eth0 inet staticiface eth0 inet static
address 192.168.0.2address 192.168.0.2
netmask 255.255.255.0netmask 255.255.255.0

14. DHCP IP SettingsDHCP IP Settings
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Redhat/CentOS:Redhat/CentOS:
vi /etc/sysconfig/network­scripts/eth0vi /etc/sysconfig/network­scripts/eth0
# Intel Corporation 82545EM Gigabit Ethernet # Intel Corporation 82545EM Gigabit Ethernet
Controller (Copper)Controller (Copper)
DEVICE=eth0DEVICE=eth0
BOOTPROTO=dhcpBOOTPROTO=dhcp
HWADDR=00:0c:29:05:e1:d2HWADDR=00:0c:29:05:e1:d2
ONBOOT=yesONBOOT=yes
TYPE=EthernetTYPE=Ethernet
USERCTL=yesUSERCTL=yes
IPV6INIT=noIPV6INIT=no
PEERDNS=yesPEERDNS=yes

15. DHCP IP SettingsDHCP IP Settings
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Ubuntu:Ubuntu:
vi /etc/network/interfacesvi /etc/network/interfaces
auto loauto lo
iface lo inet loopbackiface lo inet loopback
auto eth0auto eth0
iface eth0 inet dhcpiface eth0 inet dhcp

16. UsingUsing ifupifup andand ifdownifdown
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Only to run while settings are stored inOnly to run while settings are stored in
configuration fileconfiguration file
– ifup eth0ifup eth0
Bring up interfaceBring up interface
– ifdown eth0ifdown eth0
Shutdown interfaceShutdown interface

17. Configure Name ResolutionConfigure Name Resolution
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/etc/hosts/etc/hosts
– Static mappingStatic mapping
127.0.0.1 localhost.localdomain localhost127.0.0.1 localhost.localdomain localhost
192.168.0.1 server1.my.com server1192.168.0.1 server1.my.com server1
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/etc/resolv.conf/etc/resolv.conf
– DNS serverDNS server
search my.comsearch my.com
nameserver 192.168.0.1nameserver 192.168.0.1
nameserver 8.8.8.8nameserver 8.8.8.8

18. Configure Name ResolutionConfigure Name Resolution
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/etc/nsswitch/etc/nsswitch
– Resolving orderResolving order
hosts: files dnshosts: files dns

19. Network Diagnostic ToolsNetwork Diagnostic Tools
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ping hostping host
– Test connection availabilityTest connection availability
– Common optionsCommon options
­c ­c nn : send: send nn packets onlypackets only
­w ­w nn : set dead line in: set dead line in nn secondsseconds

20. Network Diagnostic ToolsNetwork Diagnostic Tools
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traceroute hosttraceroute host
– Display all routing nods in the routing pathDisplay all routing nods in the routing path
– Common option:Common option:
­n ­n : Skip name resolving: Skip name resolving
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mtr hostmtr host
– Advanced traceroute toolAdvanced traceroute tool

21. Network Diagnostic ToolsNetwork Diagnostic Tools
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arp -aarp -a
– Display all entries in ARP cacheDisplay all entries in ARP cache
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arp IParp IP
– Display specific entry in ARP cacheDisplay specific entry in ARP cache
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arp -d IParp -d IP
– Delete an entryDelete an entry
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arp -s IP MACarp -s IP MAC
– Set an entry manuallySet an entry manually

22. Network Diagnostic ToolsNetwork Diagnostic Tools
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host DOMAIN_NAME [DNS_SERVER]host DOMAIN_NAME [DNS_SERVER]
– Lookup A record of a domain nameLookup A record of a domain name
– Common optionsCommon options
­t ­t : query type: query type
­v ­v : verbose output: verbose output
­W ­W nn : time out in: time out in nn secondsseconds
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host IP [DNS_SERVER]host IP [DNS_SERVER]
– The reverse DNS lookupThe reverse DNS lookup

23. Network Diagnostic ToolsNetwork Diagnostic Tools
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dig DOMAIN_NAME [@DNS_SERVER]dig DOMAIN_NAME [@DNS_SERVER]
– Lookup domain name informationLookup domain name information
– Common optionsCommon options
­x IP ­x IP : reverse lookup: reverse lookup
+short +short : short answer: short answer
+trace +trace : trace recursive lookup from root down: trace recursive lookup from root down

24. Network Diagnostic ToolsNetwork Diagnostic Tools
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whois DOMAIN_NAMEwhois DOMAIN_NAME
– Lookup domain registration informationLookup domain registration information
– Common optionsCommon options
­h server ­h server : specify whois server: specify whois server

25. Network Diagnostic ToolsNetwork Diagnostic Tools
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netstatnetstat
– Display socket informationDisplay socket information
– Common optionsCommon options
­n ­n : skip domain name lookup: skip domain name lookup
­l ­l : display listening server sockets: display listening server sockets
­t ­t : display TCP sockets: display TCP sockets
­u ­u : display UDP sockets: display UDP sockets
­a ­a : display all sockets: display all sockets
­p ­p : display PID, root only: display PID, root only